Wireless local area networks (WLAN) can be used in a variety of commercial, industrial and consumer applications, thereby permitting mobile and portable user computers and devices to efficiently transmit and receive data between a user computer or device and a remote system without requiring a wired connection therebetween. Many mobile and portable users, particularly in businesses, factories, universities and other professions can benefit tremendously both in terms of efficiency and productivity with the enhanced capabilities of a WLAN.
A number of systems for implementing WLANs have been proposed and implemented. One class of systems is those conforming to, and/or interoperable with, one or more IEEE 802.11 standard. The IEEE 802.11 is a popular and well-known standard and comprises several extensions to date, with additional extensions likely. The extensions include 802.11a, 802.11b, etc. and it should be understood herein that general references to the 802.11 standard encompass the currently adopted extensions and extensions that follow.
Wireless signals conforming to the IEEE 802.11 standard propagate in a 2.4–2.5 GHz ISM (industrial, scientific and medical) band, a 5 GHz band, infrared bands and others. The ISM band in particular is currently available worldwide and generally permits unlicensed operation for spread spectrum systems. For the US and Europe, the 2,400–2,483.5 MHz band has been allocated, while for some other countries, such as Japan, another part of the 2.4–2.5 GHz ISM band has been assigned.
Networks, protocols and standards are typically designed and specified according to a now standard seven-layer ISO/OSI network model. Within that model, the 802.11 standard generally focuses on the MAC (medium access control) layer and the PHY (physical) layer.
802.11-compliant communication occurs between stations. Some stations serve as access points between a wireless medium and a distribution system other than the wireless medium, while other stations only use the wireless medium to communicate 802.11 data. An example of a distribution system is a wired local area network (LAN), such as an Ethernet-protocol LAN, the Internet, or other network. The distribution system might even be another wireless system (which might be useful to support a number of nodes that can access the access point wirelessly, but not the wireless medium that is used as that access point's distribution system). The same wireless network might also serve as the distribution system (DS) using “wireless DS” transport.
While an access point is a station according to the 802.11 standard if it interacts with the wireless medium, the term “station” is often informally used to refer to a network node that is not connected to a distribution system and the term “access point” is used to refer to a station/node that is connected to a distribution system, thus allowing a distinction between nodes that can access a distribution system outside the wireless medium and those that cannot. That convention is used hereinafter, unless otherwise indicated.
Wireless networks with multiple stations but no access points are referred to as “ad-hoc” networks. Without more, an ad-hoc network allows for communication among stations accessible via a wireless medium, but not for communications beyond that ad-hoc network.
In an 802.11 wireless network with at least one access point, a station located within a group or cell sends packets of data to the access point, which in turn forwards messages/packets/data to a destination such as a station within the same cell or, via the access point's distribution system, to a destination outside the wireless medium.
The 802.11 standard generally supports several data signalling schemes: DSSS (direct sequence spread spectrum) with differential encoded BPSK and QPSK; FHSS (frequency hopping spread spectrum) with GFSK (Gaussian FSK); OFDM (orthogonal frequency division multiplexing, infrared with PPM (pulse position modulation) are several examples. DSSS, FHSS and infrared all provide bit rates of 1 Mbs (megabits per second) and 2 Mbs. The 802.11b extension provides for a high rate CCK (Complementary Code Keying) physical layer protocol, providing bit rates of 5.5 and 11 Mbs as well as the basic DSSS bit rates of 1 and 2 Mbs within the 2.4–2.5 GHz ISM band. The 802.11a extension provides for a high bit rate OFDM (Orthogonal Frequency Division Multiplexing) physical layer protocol providing bit rates in the range of 6 to 54 Mbs in the 5 GHz band. The 802.11g extension provides for 802.11a-like signalling, but in the 2.4–2.5 GHz band.
The 802.11 basic medium access control (MAC) behavior allows interoperability between compatible physical layer protocols through the use of the CSMA/CA (carrier sense multiple access with a collision avoidance) protocol and a random back-off time following a busy medium condition. In addition, directed traffic can use an immediate positive acknowledgement (ACK frame) protocol, wherein a retransmission is scheduled by the sender if no positive acknowledgement is received. The 802.11 CSMA/CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the point in time where collisions are most likely occur. The highest probability of a collision occurs just after the medium becomes free, following a busy medium. This is because multiple stations would have been waiting for the medium to become available again. Therefore, a random back-off arrangement is used to resolve medium contention conflicts. In addition, the 802.11 MAC defines special functional behavior for fragmentation of packets, medium reservation via RTS/CTS (request-to-send/clear-to-send) polling interaction, and point coordination (for time-bounded services).
The IEEE 802.11 MAC also defines beacon frames, sent at a regular interval by an AP to allow STAs to monitor the presence of the AP. IEEE 802.11 also defines a set of management frames including probe request frames that are sent by a station and are followed by probe response frames sent by the AP. Probe request frames allow a station to actively scan whether there is an AP operating on a certain channel frequency, and for the AP to show to the station what parameter settings the AP is using.
A client uses the wireless network by finding an AP, authenticating to that AP and associating with that AP. Normally, a client associates with one AP at a time, but where connection to one AP is lost, the client can associate with another AP (or reassociate with the same one after a connection is lost or closed). The AP's of a network can communicate over a distribution system (DS). One reason for communicating between AP's is where an AP has frames buffered for a client, but loses the client. That AP might discover that the client is now associated with a different AP and will forward the buffered frames to that new AP via the DS. The access points might also connect to a network outside of the 802.11 wireless network. In some cases, the DS is not distinct from that outside network. That outside network could be another wireless network, but a common configuration has the outside network being a local area network (LAN).
When a wireless LAN station is powered on, it first looks for an access point. After it finds an access point, the wireless LAN station registers itself with the access point (authentication, association). The station can then synchronize with the access point and, thereafter, transmit and receive data frames to and from the access point. In a common example, the client station is a portable or mobile computer with a wireless networking card installed therein. 802.11 management frames are used to set up these connections.
Unlike wired networks, where a network is secured at boundaries by which wires connect to the network, wireless networks do not have well-defined boundaries. A company on one floor of a building might have a wireless network that can be reached by a computer on a different floor using a computer unrelated to the company that set up the wireless network. Consequently, it is easier to join into a wireless network, for authorized users as well as unauthorized users.
In some cases, a wireless network could be coupled to a wired network without oversight by the operators of the wired network. For example, many access points have a standard interface and can be easily plugged into a standard wired network connector, thus opening up a previously secured wired network to wireless traffic. Where an uninformed end-user replaces a wired network connection with an access point and does not secure the access point, the wired network would then be open to users within radio range of the access point, even if they were not within the physical space controlled by the organization for which the wired network is being maintained.
Some network operators have attempted to address unexpected access points by physically surveying their network. In one approach, a network administrator would walk with a network sniffer through all of the space controlled by the organization, but for large spaces, this is often impractical.
In large wireless networks, considerable effort is needed to maintain numerous access points and when a large number of access points are needed, for bandwidth reasons, coverage reasons, etc., the cost can be considerable as the full functionality of an access point needs to be repeated in the space where the network is set up.
Another difficulty of wireless networks is that of not necessarily authorized users in the authorized space. For example, if a visitor with a wireless computer or wireless device is in a company building that is covered by the company's wireless network, that visitor might connect to the company network and have access equivalent to that of an employee, and that is generally undesirable.
Yet another difficulty of wireless networks is network overlap. Where a space is to have multiple wired networks, parallel sets of network cabling can be laid down. This can be effected with wireless networks by overlapping access points and programming the access points to be selective with associations, but this necessarily involves more hardware than is necessary to support the space.
In light of the above, the inventors have invented improvements to wireless networks.